This invention relates to DC power supplies and in particular, but not solely, power supplies for whiteware appliances.
Modern whiteware appliances commonly use switched mode power supplies to supply power to motors, solenoids and control electronics. Such power supplies consume power even when on standby (a condition which the user considers as being switched off). Further, switched mode power supplies require the use of relatively expensive components such as inductors and capacitors having a high working voltage. In such whiteware appliances it is common to control motor torque and speed using pulse width modulation (PWM) techniques to control current supplied to the motor windings. Such PWM circuits also require relatively expensive components.
It is an object of the present invention to provide a power supply which goes at least some way towards overcoming the abovementioned disadvantages.
Briefly and in accordance with the foregoing, a variably controlled output voltage DC power supply is provided and is particularly suited for use in whiteware appliances having built-in heater elements. The power supply has a standby mode and includes a rectifier, a reservoir capacitor, a semiconductor switching device, a voltage dropping resistor and a microprocessor. The rectifier receives AC from a mains supply. In use, the reservoir capacitor provides the DC output of the power supply. The semiconductor switching device is in series with the reservoir capacitor and controls current flow from the output of the rectifier to the reservoir capacitor. The voltage dropping resistor is connected in series with the rectifier which when the supply is in the ON mode and reduces the peak unidirectional voltage provided from the rectifier to a level compatible with the DC output voltage of the supply. The microprocessor variably controls the conduction angle of the switching device to vary the DC voltage across the reservoir capacitor to a desired value.